The present invention pertains to a bandpass filter and more specifically to a capacitively coupled bandpass filter which is operable in the upper UHF region and which utilizes stepped impedance resonators.
The design of bandpass filters in the UHF region requires numerous tradeoffs with respect to circuit topology, bandwidth, element realization, allowable insertion loss, etc. before a design is arrived at that will satisfy all of the desired constraints. In many cases, the design of a conventional filter is driven by the realizable range of inductor values for the frequency range of interest. In the case of wide band or narrow band bandpass filter designs, the required values of the inductors and capacitors diverge rather quickly from the case of the standard low pass to bandpass transformation, which ends up with a series LC, shunt LC configuration. A well-known technique to handle the narrow band bandpass filter designs is the capacitively coupled bandpass filter which introduces a number of redundant elements such that all components can be maintained well within their realizable ranges. The approach is well suited for bandwidths less than about 20% of center frequency and allows the designer the flexibility of choosing realizable inductor values to be used in the circuit. An example of a prior art capacitively coupled bandpass filter using shunt LC resonator elements is disclosed in Watkins, LR, "Narrow band Butterworth or Chebyshev Filter Using the TI-59 Calculator," RF Design, Nov./Dec. 1980, pp. 22-32.
However, a problem arises in the upper UHF region (greater than around 500 megahertz) with respect to the lumped inductor values While realizable values can be chosen, implementation becomes very difficult. When values are chosen that make it possible to construct inductors, air core inductors are warranted and a considerable amount of adjustment is normally required to obtain the desired performance. This, in turn, drives up the cost of the associated filter Additionally, the parasitic elements inherently present in a lumped element filter realization, due to board capacitance or non-ideal components, are hard to account for, and in many cases result in a filter with less than optimum performance, especially in the area of out-of-band rejection or isolation. Self-resonance of the lumped element components also causes spurious responses to occur at various frequencies outside the passband, thus limiting the frequency of spurious free operation. The frequency of the spurious responses is also very hard to predict. In addition, lumped element filters exhibit crosstalk between components. This crosstalk results in lower achievable isolation. While methods are available to improve isolation for lumped element filters, such as walls between components, such methods are somewhat expensive and time consuming.
A stepped impedance resonator (SIR) element is known to those skilled in the art. An example of a prior art comb-line filter using stepped impedance resonators is disclosed in Makimoto, M. and Yamashita, S. "Compact Bandpass Filters Using Stepped Impedance Resonators." Proceedings of the IEEE, Vol. 67, No. 1, Jan. 1979, pp. 16-19. This article describes the use of the SIR element in a comb-line filter and is based on cavity techniques. The comb-line filter using SIR elements in the Makimoto article, if it were done using printed circuit techniques, it would be limited to very narrow band filters due to the distributed element coupling that is utilized. An additional problem with distributed elements is that a great deal of space is used.